The core-end of a large electric machine contains a stator core-end and a rotor core-end. The stator core-end (“core-end”) contains several parts, such as end-stepped laminations, press-finger, clamping ring, flux shield etc. plus a prevailing stator end winding. The rotor core-end also contains several parts, such as a magnetic spindle, a retaining ring, a centering ring etc. plus a prevailing rotor end winding. This prevailing rotor end winding comprises plurality of rotor end-turns, separated by plurality of interspaces, and is laid over the magnetic spindle, just outside the core. The prevailing stator end winding comprises several pairs of stator end-turns separated by interstices. All these rotor end-turns and stator end-turns are rectangular conductors that carry large DC and AC currents respectively. Their magnetic fields hit the core-end, causing core-end heating, so are the subject matter of the present disclosure.
The magnetic field produced by the prevailing rotor end winding has a large axial component that hits the large resistive surfaces in the core-end at 90°. The prevailing stator end winding can superpose an additional axial component as discussed in Paras [0047] to [0050]. Changes in the net axial magnetic field induce an eddy voltage in all resistive core-end parts according to Faraday's law. This eddy voltage induces large eddy current loops in them, which dissipate in the resistive bodies to produce core end heating. If this eddy voltage exceeds the inter-laminar insulation breakdown voltage, it can short defects in the end-stepped laminations, creating undetectable hot spots. This additional heat sharply increases the local temperature at the core-end, reducing the efficiency and power rating of the machine. The power rating is often limited to prevent core-end overheating, viz., exceeding specified thermal limits.
In prior art, several devices have therefore been developed to reduce core-end heating. These proven devices include, core-end stepping, flux shields, flux-shunts, shorter rotors, slitted teeth (“pistoye slots”), resistive or laminated clamping rings, low-loss or easy-axis steels, thicker insulation, complex vent ducts, etc. as reviewed in U.S. Pat. No. 8,203,249.
The core-end stepping trims the teeth at the core-end, in several steps in staircase fashion, to increase the magnetic gap locally. These teeth may also be slitted to reduce eddy currents further. Typical end-step profiles are shown in U.S. Pat. Nos. 2,795,714; 4,208,597; 6,455,977; 6,525,444; 6,688,136 and 7,265,473. The increased reluctance reduces the strength of the magnetic field locally, thereby reducing electric field, hence eddy-heat. Core-end stepping also causes the spot flux density at the tooth tip to fall below saturation, thereby discouraging formation of local hot spots. Even though very popular, the core-end stepping has several drawbacks.
For example, higher excitation, required by the increased reluctance, lowers the overall efficiency per U.S. Pat. No. 6,525,444. The core-end stepping reduces the clamping pressure on the teeth, causing flutter and machine failure per U.S. Pat. No. 7,057,324. Mecrow in 1989 suggested that core-end stepping might have limited success in reducing the axial flux under load, casting doubts on its efficacy. Too deep end-stepping can damage the machine per Maughan 2013. Optimal end-step profile often requires expensive three-dimensional magnetic field simulations. The shearing/slitting needed to make end-stepped laminations can degrade their magnetic quality. The sharp end-steps can also produce coolant-starved zones that could cause hot spots. A flux shield is still needed to prevent overheating of press-fingers, clamping rings etc.
In view of these numerous drawbacks, it is desirable to develop an alternative device that can reduce core-end heating without core-end stepping. This disclosure presents such alternative device, termed Stealth End Winding. It can potentially replace alternative devices such as the flux shield, pistoye slots, complex vent ducts, expensive steel etc. that are presently used to reduce core-end heating. Elimination of all these expensive parts makes the Stealth End Winding commercially attractive.
FIG. 1 shows a prevailing core-end in a large electric machine of prior art. It comprises a stator core-end 60 and a rotor core-end 64. The stator core-end 60 (“stepped core-end”) comprises end-stepped laminations 62, a press-finger 66, a clamping ring 68 and vent ducts 58 etc. An optional flux shield 69 is shown by dashed lines. The prevailing stator end winding is not shown for clarity. The rotor core-end 64 is made of a magnetic rotor 20 with a stepped down portion termed magnetic spindle 44 (hatched area) that faces the stator core-end 60. The step-down 27 on the rotor is optionally aligned with the last core-end lamination CFT as shown. The magnetic spindle 44 supports the prevailing rotor end winding. The prevailing rotor end winding is synonymous to a plurality of rotor end-turns. Five rotor end-turns 31-35, shown as dark-shaded rectangles, separated by plurality of interspaces 67, illustrate the rotor end winding. They carry DC currents into the plane of the paper. They are cradled in a cavity formed by the step-down 27, the magnetic spindle 44, a centering ring 45 and a non-magnetic retaining ring (not shown).
FIG. 2 details a single rotor end-turn 50 (“end-turn”), which is an extension of a field turn 24 outside its slot 22. It comprises plurality of flat conductors which are not shown for clarity. Each end-turn 50 comprises a straight-turn 26 and an arc-turn 30. The straight-turn 26 portion of the end-turn 50 is straight, axial and protrudes out of the step-down 27. It produces magnetic field lines 42 in a plane parallel to the cross-section of the rotor. For clarity, this disclosure shows only portions of the magnetic field lines that flow through air. The magnetic field lines 42 obviously do not hit the core-end 60. Hence, the straight-turn 26 does not cause core-end heating.
The arc-turn 30 portion of the end-turn 50 is a circular arc over the magnetic spindle 44, subtending an angle at the center. The arc-turn 30 has a rectangular cross-section 31 with an outer radius 38 and an inner radius 39 (FIG. 4A). The DC field currents flowing through arc-turn 31 produce magnetic field lines 40 in a plane that is parallel to a longitudinal section of the rotor. It is clear that such magnetic field lines 40 do hit the core-end. So the terms rotor end-turn, its rectangular cross-section, and their labels are used loosely to refer to an arc-turn of a rotor end winding.
FIG. 3 shows the net magnetic field lines 40 produced by the prevailing rotor end winding alone, viz., from all arc-turns 31-35. Such net magnetic field lines 40 hit the end-stepped laminations 62, press-fingers 66, clamping ring 68, flux shield 69 etc and return back via stator core-end 60 and magnetic spindle 44 to close the loop around the arc-turns 31-35. When the rotor rotates, their changing axial component induce electric field per Faraday law. This electric field causes eddy currents in all the core-end parts, which in turn generates large eddy heat. This establishes that the arc-turns 31-35 are a major source of core-end heating.